Spin density in a triazole-nitronyl-nitroxide radical presenting linear ferromagnetic interactions: role of hydrogen bonding

The compound 2-{3-[4-methyl-1,2,4-triazolyl]}-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide, abbreviated as Metrz-Nit, crystallizes in the non-centrosymmetric space group P2(1)2(1)2(1). The investigation of the magnetic properties has revealed the occurrence of intermolecular ferromagnetic interactions. The crystal structure has been refined by neutron diffraction at 11 K. The spin density distribution has been determined from polarized neutron diffraction experiments carried out at 1.5 K under a magnetic field of 5 T. As expected, the main contributions of the spin distribution have been observed in the 2p(pi) orbitals of the nitrogen and oxygen atoms of the two NO groups, and a significant negative spin population has been detected on the sp(2) carbon atom of the nitronyl nitroxide moiety. The spin distribution is slightly dissymmetrical, so that the sp(3) carbon atoms in alpha-position of the nitro nitrogen atoms carry spin populations of opposite signs. Concerning the triazole ring, the main spin population, of negative sign, has been found on the nitrogen atom occupying the 3-position. The carbon atom of the methyl group attached to the 4-position has been also found to carry a significant negative spin population. The spin populations on the hydrogen atoms have been determined. These experimental data have been compared to the results of LSD calculations performed on an isolated molecule. The role of intermolecular interactions in the spin distribution has been discussed. (C) 1999 Elsevier Science B.V. All rights reserved.